Specializations

Research Summary

A. Synthetic Enzymes and Redox Catalysts

Diorganoselenides and tellurides undergo reversible two-electron redox processes based on oxidative-addition and reductive-elimination catalysts. These cycles are the basis for the construction of synthetic enzymes which mimic horseradish peroxidase or the haloperoxidases for the activation of hydrogen peroxide, and provide templates for chiral halogenations, and templates for the kinetic resolution of enantiomeric mixtures of vicinal dihalides through dehalogenation reactions, which we have examined in the Department of Chemistry at the University at Buffalo. These reactions can be used as “green” approaches to the halogenations of organic substrates by avoiding the use of elemental halogens and, instead, using hydrogen peroxide (which decomposes to water and oxygen) and halide salts (chloride, bromide, iodide) to generate the elemental halogen or hypohalous acid. The organochalcogenides catalyze the process.

The organochalcogen catalysts can be sequestered in siloxane-based xerogels to give catalysts on a solid support. In several examples, the catalytic activity has been increased by nearly two orders of magnitude relative to the catalytic activity in solution. The xerogel supports can be tailored for halide permeability as well as peroxide permeability to improve catalytic efficiency. The sequestered catalysts can be recycled through several catalytic cycles. Ionic liquids also accelerate the catalytic efficiency of the reactions with organochalcogen catalysts. We are currently investigating the synthesis and properties of organochalcogen catalysts incorporating imidazolium functionality to mimic the ionic liquids. These derivatives include materials that can b attached covalently to the xerogels.

The organochalcogen catalyst-xerogel sol can also be cast as a thin film to provide an antifouling/foul-release surface in a marine environment. Seawater in sheltered areas contains micromolar concentration of hydrogen peroxide near the surface from the photochemical decomposition of organic matter and from rain water, which interacts with lightning to produce hydrogen peroxide in the rain drops. Several different kinds of bacteria also produce hydrogen peroxide on a submerged surface. Local peroxide concentrations can be as high as 50 micromolar. In the presence of the xerogel-sequestered catalyst, the reaction of peroxide with the halide salt (0.5 M in chloride, 1 mM in bromide, and 1 micromolar in iodide in seawater) is accelerated producing a monolayer of bleach/hypohalous acid on the xerogel surface. Settlement of marine organisms is thus discouraged.

The xerogel can also be tailored to have a surface energy that is in the zone of minimal bioadhesion on the so-called Baier Curve to provide surfaces that release fouling organisms. We are currently exploring surfaces of appropriate surface energy that also have micrometer and nanometer-scale topological features, which can serve to minimize adhesion and optimize release. The topological features are observed using AFM microscopy and the infrared microscope. We have had recent success in providing surfaces that release barnacles and macro-fouling algae. This work is currently funded by the Office of Naval Research.

The rhodamine dyes and analogues incorporating the heavier chalcogen atoms sulfur, selenium, and tellurium interact with transmembrane domains of the ABC transporter P-glycoprotein. These molecules can act as either strong stimulators of ATPase activity in the protein or as inhibitors of ATPase activity. We have recently shown that the interconversion of an amide to a thioamide gives several predictable changes: 1) The amides are ATPase stimulators while the thioamides are ATPase inhibitors. 2) The amides are transported rapidly by P-glycoprotein while the thioamides are transported extremely slowly. 3) Both the amides and thioamides interact with P-glycoprotein in such a way that calcein AM is transported rapidly into the cell.

Crosslinking studies with David Clarke’s group at the University of Toronto suggest that the rhodamines bind to the “open” conformation of P-glycoprotein. In contrast, the chalcogenopyrylium inhibitors appear through crosslinking studies to bind most strongly to the closed conformation of P-glycoprotein. Amide and thioamide pairs in the chalcogenopyrylium series have nearly identical response characteristics to the chalcogenorhodamines.

Our goals in this project are to design sub-nanomolar inhibitors of P-glycoprotein (in collaboration with Tom Raub/Geri Sawada at Eli Lilly), to co-crystallize rhodamine and/or pyrylium dyes with crystals of the “inward” and “outward” facing conformations of P-glycoprotein in order to provide a high-resolution crystal structure (in collaboration with Dr. Geoffrey Chang of Scripps), and to elucidate mechanistic details of the ATP/ADP cycle as well as details of the biochemistry of the protein (in collaboration with Dr. Tom Raub/Geri Sawada at Eli Lilly, in collaboration with Dr. Greg Tombline at the University of Rochester, and in collaboration with Dr. David Clarke at the University of Toronto). We are also examining the interaction of the chalcogenorhodamine and chalcogenopyrylium dyes with other ABC transporters including MRP1 and BCRP in collaboration with Dr. Susan Cole at Queens University. The work is currently funded through support from the NIH.

D. Organic Dyes as Photosensitizers for the Production of Solar Electricity and Solar Hydrogen

Chalcogenorhodamine and chalcogenopyrylium dyes are efficient photosensitizers for the harvesting of photons, especially in the 400-800-nm window where solar light is quite intense. We have demonstrated that carboxylic acid anchors on these dyes are effective for binding the materials to titanium dioxide, where ICPE values are quite high. The formation of H-aggregates improves values of IPCE as well as injection yields into titania relative to unaggregated dyes. The carboxylic acid anchors are not effective for producing long-lived devices. We have been preparing chalcogenorhodamine and chalcogenopyrylium dyes with phosphonate anchors, which give much tighter binding to titania without sacrificing IPCE. Long-lived devices have been prepared from these materials with efficiencies of 1-2%. This work is being done in collaboration with Dr. David Watson of UB and is currently funded by a grant from the PRF.

Among our goals for this project are to demonstrate that organic photosensitizers are practical for use in solar applications and that sensitization across a broad portion of the 400-700-nm window is possible with high values of IPCE. Several of our dyes have higher values of IPCE than the ruthenium complexes over this window. The design of appropriate molecules also allows the controlled formation of H- and/or J-aggregates, which we have demonstrated improves injection yields. We have also demonstrated that both carboxylic acid and phosphonic acid anchors can be used without sacrificing performance.

The chalcogenorhodamine and chalcogenopyrylium dyes are also efficient photosensitizers for the production of solar hydrogen. In homogeneous systems using cobalt complexes as catalysts, turnover numbers as high as 10,000 moles of hydrogen per mole of catalyst have been realized with an overall efficiency of > 30%. These dyes are also efficient photosensitizers for colloidal platinum and palladium catalysts and for platinized titania. This work is being done in collaboration with Prof. Rich Eisenberg at the University of Rochester.

E. New Registration Systems for Biosensing Applications

The ultimate purpose for this research is to develop a new generation of biosensing/registration platforms for real-world sensing applications in a collaborative effort between our group and Prof. Frank Bright’s group in the Department of Chemistry at the University at Buffalo. There have been numerous biosensing/bioassay platforms developed over the years to detect and quantify analytes in complex samples. Unfortunately, all current systems are limited because they are not universal, suffer from background signals that bias the measurements, and/or they are somewhat difficult to manufacture. We propose an entirely new approach to circumvent the aforementioned problems and aim to develop a new generation of biosensing/registration platforms that exploit the tailored recognition/switching dye chemistry pioneered by the Detty group and the Bright group’s expertise in sol-gel-derived composite materials, analytical biosensing, protein biophysics, and optical spectroscopy.